This present invention relates to burnable poison rods for use in a nuclear reactor and to a fuel assembly of a nuclear reactor containing such rods.
It is well-known that the process of nuclear fission involves the disintegration of the fissionable nuclear fuel material into two or more fission products of lower mass number. Among other things the process also includes a net increase in the number of available free neutrons which are the basis for a self-sustaining reaction. When a reactor has operated over a period of time the fuel assembly with fissionable material must ultimately be replaced due to depletion. Inasmuch as the process of replacement is time consuming and costly, it is desirable to extend the life of a given fuel assembly as long as practically feasible. For that reason, deliberate additions to the reactor fuel of parasitic neutron-capturing elements in calculated small amounts may lead to highly beneficial effects on a thermal reactor. Such neutron-capturing elements are usually designated as "burnable poisons" or "burnable absorbers" if they have a high probability (or cross section) for absorbing neutrons while producing no new or additional neutrons or changing into new absorbers as a result of neutron absorption. During reactor operation the burnable absorbers are progressively reduced in amount so that there is a compensation made with respect to the concomitant reduction in the fissionable material.
The life of a fuel assembly may be extended by combining an initially larger amount of fissionable material as well as a calculated amount of burnable absorber. During the early stages of operation of such a fuel assembly, excessive neutrons are absorbed by the burnable absorber which undergoes transformation to elements of low neutron cross section which do not substantially affect the reactivity of the fuel assembly in the latter period of its life when the availability of fissionable material is lower. The burnable absorber compensates for the larger amount of fissionable material during the early life of the fuel assembly, but progressively less absorber captures neutrons during the latter life of the fuel assembly, so that a long life at relatively constant fission level is assured for the fuel assembly. Accordingly, with a fuel assembly containing both fuel and burnable absorber in carefully proportioned quantity, an extended fuel assembly life can be achieved with relatively constant neutron production and reactivity.
Burnable absorbers which may be used include boron, gadolinium, samarium, europium, and the like, which upon the absorption of neutrons result in isotopes of sufficiently low neutron capture cross section so as to be substantially transparent to neutrons.
The incorporation of burnable absorber in fuel assemblies has been recognized in the nuclear fuel as an effective means of increasing fuel capacity and thereby extending core life. Burnable absorbers are used either uniformly mixed with the fuel (i.e., distributed aborber) or are placed discretely as separate elements in the reactor, so arranged that they burn out or are depleted at about the same rate as the fuel. Thus, the net reactivity of the core is maintained relatively constant over the active life of the core.
When the burnable absorbers are placed as discretely separate elements in the reactor, the same are normally contained in a burnable poison rod, and the rods inserted into empty control rod guide thimbles in the fuel assembly. Control rods are not required in all guide thimbles of all fuel assemblies, thus allowing for the use of burnable poison rods.
In U.S. Pat. No. 4,342,722, there is described prior art burnable poison rods and a specific rod is disclosed which contains sections of boron glass. That rod contains a plurality of sections of boron glass tubes, and provides for the joints between the tube sections to be outside the zone of maximum flux density. The rod described is, however, of a full length design, such rods being on the order of 8-14 feet in length.
Situations arise where a part length burnable poison rod is desirable. In such part length rods, the neutron absorber must be reduced in amount and the absorber must be repositioned near the center of the core height. One proposed design of a part length burnable poison rod is a shortened version of a full length burnable poison rod containing a shortened length of burnable absorber. This design, due to its shorter length, however, is not compatible with present handling equipment. If costly handling equipment modifications are to be avoided, a part length burnable poison rod is needed which has the dimensions of conventional full length burnable poison rods.